Egg Spinach Mushroom Casserole

Picture a golden, gently puffed casserole emerging from the oven—cheddar stretching into tender strands, earthy mushrooms glistening, and a peppery, garlicky aroma mingling with sweet onion and fresh herbs.

Imagine the first slice: silky eggs cradling juicy cremini and just-wilted spinach, edges lightly bronzed, each bite cozy yet vibrant.

This is my kind of comfort—nourishing, unfussy, and endlessly adaptable—because breakfast should feel like a hug and still get you out the door on time.

This casserole became my week-saver the morning after a late soccer game; I assembled it the night before, baked while we packed lunches, and everyone ate well without a scramble.

It’s ideal for busy weeknights when you’re craving something wholesome, for Sunday suppers with a salad on the side, or for hosting brunch without hovering over a skillet.

Make-ahead friendly, sturdy for leftovers, and always crowd-pleasing.

Ready? Let’s cook!

Why You’ll Love It

• Delivers savory, cheesy flavor with tender mushrooms and fresh herbs
• Packs protein-rich eggs for a satisfying, energizing breakfast
• Assembles easily with everyday ingredients and simple steps
• Bakes ahead-friendly; refrigerate overnight for stress-free mornings
• Reheats gently, staying moist and delicious for leftovers

Ingredients

• 1 tbsp olive oil — for sautéing (use good extra-virgin)
• 1 tbsp unsalted butter — adds richness (prefer high-fat European style)
• 1 medium yellow onion, diced — base aromatics (choose sweet, firm bulbs)
• 3 cloves garlic, minced — fragrant depth (use fresh, not jarred)
• 8 oz cremini mushrooms, sliced — meaty bite (brown well for flavor)
• 6 oz fresh spinach, roughly chopped — greens (baby spinach wilts fastest)
• 10 large eggs — the custard base (use fresh, pasture-raised if possible)
• 1 cup whole milk — creaminess (cold helps whisk smoothly)
• 1/2 cup heavy cream — richer set (aim for 36% fat)
• 1 cup shredded cheddar cheese — melty tang (sharp cheddar melts best)
• 1/2 cup grated Parmesan cheese — savory boost (freshly grated is best)
• 1 tsp kosher salt — seasoning (Diamond Crystal preferred)
• 1/2 tsp black pepper — gentle heat (freshly ground)
• 1/2 tsp smoked paprika — subtle smoke (Spanish pimentón if available)
• 1/4 tsp crushed red pepper flakes (optional) — mild kick (adjust to taste)
• 2 tbsp fresh parsley, chopped — brightness (flat-leaf preferred)
• 1 tbsp fresh chives, chopped — oniony lift (add freshness)
• 1 tbsp olive oil — for greasing (lightly coat baking dish)

Step-by-Step Method

Preheat & Prep the Dish

Preheat oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish with olive oil. Set a rack in the center of the oven. Gather equipment and ingredients. Position a cooling rack or trivet for later. This prep guarantees an even bake and easy release. Keep ingredients measured and ready for quick assembly.

Sauté the Aromatics

Heat olive oil and butter in a large skillet over medium heat until foamy. Add diced onion and cook 3 to 4 minutes until translucent. Stir in minced garlic and cook 30 seconds until fragrant. Avoid browning the garlic. This step builds a savory base that infuses the casserole with depth.

Brown the Mushrooms

Add sliced cremini mushrooms to the skillet. Cook 6 to 8 minutes, stirring occasionally, until they release moisture and develop golden edges. Season lightly with a pinch of salt to draw out liquid. Cook off excess moisture fully to prevent a watery casserole. Aim for a concentrated, savory mushroom flavor.

Wilt the Spinach

Stir in chopped spinach and cook 1 to 2 minutes until just wilted. Toss to combine evenly with mushrooms and aromatics. Remove the skillet from heat to stop cooking. Taste and adjust seasoning lightly if needed. Spread mixture on a tray briefly to steam off extra moisture, if very wet.

Whisk the Egg Base

In a large mixing bowl whisk eggs, whole milk, and heavy cream until smooth and fully combined. Break up any streaks of yolk or white. Whisk in kosher salt, black pepper, smoked paprika, and crushed red pepper flakes if using. Aim for a homogenous, aerated mixture for a tender, custardy set.

Fold in the Cheeses & Herbs

Fold cheddar, Parmesan, chopped parsley, and chives into the egg mixture. Mix gently to distribute evenly without overworking. Guarantees cheeses are well dispersed for consistent flavor and pockets of melt. Adjust seasoning if desired. Keep the mixture fluid and smooth, not overly thick.

Layer the Vegetables

Spread the mushroom-spinach mixture evenly in the prepared baking dish. Distribute in a level layer to avoid dense pockets. This guarantees even baking and uniform slices. Lightly press to settle. The vegetables form the flavorful base that the egg mixture will permeate as it bakes.

Pour & Smooth the Custard

Pour the egg-cheese mixture over the vegetables. Tilt or gently nudge with a spatula to settle and distribute ingredients evenly. Avoid stirring in the dish, which can cause uneven baking. Remove any large bubbles. The surface should be level for an even rise and set.

Bake Until Just Set

Bake 28 to 35 minutes, rotating once if needed, until the center is set with a slight jiggle and a knife near the middle comes out clean. Avoid overbaking to keep it tender and moist. The casserole will continue to firm as it rests. Watch closely near the end.

Rest, Slice & Serve

Remove from the oven and rest 10 minutes on a cooling rack to set the custard and make cleaner slices. Cut into squares using a sharp knife. Serve warm. Garnish with extra chives or parsley if desired. Refrigerate fully cooled leftovers and reheat gently at 325°F to preserve texture.

Ingredient Swaps

• Dairy-free: replace butter with olive oil; swap milk + cream with 1½ cups unsweetened almond or oat milk plus 2 tablespoons olive oil; use dairy-free cheddar/Parmesan or 3 tablespoons nutritional yeast.
• Lower fat: use all 2% milk (1½ cups) instead of milk + cream; reduce cheese by half.
• Vegetarian protein boost: add 1–1½ cups cooked lentils or diced, sautéed tofu; or swap 4 eggs with 1½ cups liquid egg whites.
• Mushroom swap: use button, portobello, or oyster mushrooms; or sub zucchini or bell peppers if mushrooms aren’t available.
• Greens swap: kale, Swiss chard, or arugula (sauté kale/chard a few extra minutes).
• Cheese options: mozzarella, Monterey Jack, Gruyère, or feta for cheddar; Pecorino Romano for Parmesan.
• Allium alternatives: leeks or shallots for onion; 1 teaspoon garlic powder if fresh garlic is unavailable.
• Spice/herb tweaks: sweet paprika for smoked; omit red pepper or use a pinch of cayenne; parsley/chives can be swapped with dill, basil, or green onions.
• Budget-friendly: skip Parmesan and use only cheddar; use frozen spinach (thawed and well-squeezed).
• Regional: add roasted poblanos and pepper jack (Tex-Mex); sun-dried tomatoes and oregano with feta (Mediterranean).

You Must Know

Doneness • If the center still wobbles like loose custard after the set time, tent with foil and continue 3–5 minutes until the middle has a gentle jiggle and a knife comes out with moist crumbs, not liquid—carryover will finish setting in the 10-minute rest.

Troubleshoot • When you see pooling liquid around edges after cutting, return slices to the hot pan or a 325°F oven for 5–8 minutes; excess moisture usually comes from under-dried mushrooms/spinach—heat drives off remaining water and re-firms the custard.

Scale • For 8–10 servings in a 9×13 that’s taller, increase eggs to 12, milk to 1¼ cups, cream to ¾ cup, cheeses by 25%, and extend time by 8–12 minutes; visual cue is domed, lightly golden top with a faint center jiggle.

Flavor Boost • For deeper savoriness, add 1 teaspoon soy sauce or ½ teaspoon fish sauce to the egg base and ½ teaspoon Dijon; it amplifies umami without tasting “saucy”—taste the custard base before pouring to ensure it’s pleasantly seasoned.

Make-Ahead • To serve straight from the fridge for brunch, cover the assembled casserole and chill up to 18 hours; before cooking, allow a 20–30 minute counter warm-up, then add 5–10 minutes to the oven time—look for 165°F in the center on an instant-read thermometer.

Serving Tips

• Serve with toasted sourdough and a simple arugula-lemon salad.
• Top slices with avocado, cherry tomatoes, and a dollop of Greek yogurt.
• Pair with crispy bacon or smoked salmon and fresh fruit.
• Garnish with extra chives, paprika, and a drizzle of chili oil.
• Plate alongside roasted breakfast potatoes and a citrus mimosa.

Storage & Make-Ahead

Refrigerate leftovers in an airtight container for up to 4 days.

Cool completely before storing.

Reheat gently at 325°F until warmed through.

For make-ahead, assemble through pouring the egg mixture, cover, and refrigerate overnight.

Add 5–10 minutes to bake time.

Freezes well up to 2 months.

Thaw overnight before reheating.

Reheating

Reheat gently.

Microwave covered at 50% power in short bursts.

Oven at 325°F covered until warm.

Stovetop low heat with lid, a splash of milk or water to prevent drying.

Church Potluck Favorite

Leftovers reheat like a charm, but this casserole really shines when I carry it, still warm, into a church potluck and set it on the table with a stack of plates.

I watch heads lift at the buttery-onion aroma and that golden cheddar lid.

You don’t need fuss—just a sharp knife, a steady hand, and a confident slice.

Here’s what guests notice first:

1. Steam curling up, carrying garlic, paprika, and earthy mushroom.
2. Sunlit squares speckled with parsley and chives.
3. Custardy centers that wobble slightly, then settle.
4. Crisped edges inviting first bites.

I set a serving spoon, announce it’s vegetarian, and step back.

The pan lightens fast.

If you’re feeding a crowd, bake two.

It travels well, slices clean, and satisfies early and often.

Final Thoughts

Ready to dig in? Give this cozy casserole a try, and feel free to tweak it with your favorite add-ins—like crispy bacon, roasted peppers, or extra cheese—to make it your own!

Frequently Asked Questions

Can I Freeze the Casserole Unbaked and Bake From Frozen?

Yes—you can freeze it unbaked. I wrap it tightly, freeze up to 2 months, then bake from frozen at 350°F, loosely covered, about 60–75 minutes. Uncover to brown, and listen for that gentle, set-center whisper.

How Do I Prevent the Casserole From Puffing and Collapsing?

Keep the bake gentle and steady: I lower oven temp slightly, avoid overwhipping eggs, don’t slam doors, and pull it when barely set. Let it rest ten minutes; the steam settles, structure firms, and no dramatic deflation.

What’s the Best Pan Material: Glass, Ceramic, or Metal?

Use metal for quickest, most even bake; I reach for light-colored aluminum. Glass browns edges more, runs hot; ceramic’s gentle but slower. If using glass or ceramic, lower temp 25°F and extend time until center just set.

How Do I Scale the Recipe for Different Pan Sizes?

Use area ratios: 9×13=117 sq in. I scale ingredients proportionally to your pan’s area and bake to doneness. For 8×8 (64), I halve; 9-inch round (64), same; 9×9 (81), two-thirds. Watch center’s jiggle.

Can I Make It Dairy-Free Without Sacrificing Texture?

Yes—you can. I swap milk/cream with full-fat coconut milk, use vegan cheddar/Parm, and add a tablespoon of cornstarch. I sauté mushrooms dry, then oil. The bake sets custardy, savory, and lush—no one misses dairy.